1 // SPDX-License-Identifier: GPL-2.0 2 /* 3 * Copyright (C) 2022-2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved. 4 */ 5 6 #include <linux/array_size.h> 7 #include <linux/minmax.h> 8 #include <vdso/datapage.h> 9 #include <vdso/getrandom.h> 10 #include <vdso/unaligned.h> 11 #include <asm/vdso/getrandom.h> 12 #include <uapi/linux/mman.h> 13 #include <uapi/linux/random.h> 14 15 #undef PAGE_SIZE 16 #undef PAGE_MASK 17 #define PAGE_SIZE (1UL << CONFIG_PAGE_SHIFT) 18 #define PAGE_MASK (~(PAGE_SIZE - 1)) 19 20 #define MEMCPY_AND_ZERO_SRC(type, dst, src, len) do { \ 21 while (len >= sizeof(type)) { \ 22 __put_unaligned_t(type, __get_unaligned_t(type, src), dst); \ 23 __put_unaligned_t(type, 0, src); \ 24 dst += sizeof(type); \ 25 src += sizeof(type); \ 26 len -= sizeof(type); \ 27 } \ 28 } while (0) 29 30 static void memcpy_and_zero_src(void *dst, void *src, size_t len) 31 { 32 if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)) { 33 if (IS_ENABLED(CONFIG_64BIT)) 34 MEMCPY_AND_ZERO_SRC(u64, dst, src, len); 35 MEMCPY_AND_ZERO_SRC(u32, dst, src, len); 36 MEMCPY_AND_ZERO_SRC(u16, dst, src, len); 37 } 38 MEMCPY_AND_ZERO_SRC(u8, dst, src, len); 39 } 40 41 /** 42 * __cvdso_getrandom_data - Generic vDSO implementation of getrandom() syscall. 43 * @rng_info: Describes state of kernel RNG, memory shared with kernel. 44 * @buffer: Destination buffer to fill with random bytes. 45 * @len: Size of @buffer in bytes. 46 * @flags: Zero or more GRND_* flags. 47 * @opaque_state: Pointer to an opaque state area. 48 * @opaque_len: Length of opaque state area. 49 * 50 * This implements a "fast key erasure" RNG using ChaCha20, in the same way that the kernel's 51 * getrandom() syscall does. It periodically reseeds its key from the kernel's RNG, at the same 52 * schedule that the kernel's RNG is reseeded. If the kernel's RNG is not ready, then this always 53 * calls into the syscall. 54 * 55 * If @buffer, @len, and @flags are 0, and @opaque_len is ~0UL, then @opaque_state is populated 56 * with a struct vgetrandom_opaque_params and the function returns 0; if it does not return 0, 57 * this function should not be used. 58 * 59 * @opaque_state *must* be allocated by calling mmap(2) using the mmap_prot and mmap_flags fields 60 * from the struct vgetrandom_opaque_params, and states must not straddle pages. Unless external 61 * locking is used, one state must be allocated per thread, as it is not safe to call this function 62 * concurrently with the same @opaque_state. However, it is safe to call this using the same 63 * @opaque_state that is shared between main code and signal handling code, within the same thread. 64 * 65 * Returns: The number of random bytes written to @buffer, or a negative value indicating an error. 66 */ 67 static __always_inline ssize_t 68 __cvdso_getrandom_data(const struct vdso_rng_data *rng_info, void *buffer, size_t len, 69 unsigned int flags, void *opaque_state, size_t opaque_len) 70 { 71 ssize_t ret = min_t(size_t, INT_MAX & PAGE_MASK /* = MAX_RW_COUNT */, len); 72 struct vgetrandom_state *state = opaque_state; 73 size_t batch_len, nblocks, orig_len = len; 74 bool in_use, have_retried = false; 75 void *orig_buffer = buffer; 76 u64 current_generation; 77 u32 counter[2] = { 0 }; 78 79 if (unlikely(opaque_len == ~0UL && !buffer && !len && !flags)) { 80 struct vgetrandom_opaque_params *params = opaque_state; 81 params->size_of_opaque_state = sizeof(*state); 82 params->mmap_prot = PROT_READ | PROT_WRITE; 83 params->mmap_flags = MAP_DROPPABLE | MAP_ANONYMOUS; 84 for (size_t i = 0; i < ARRAY_SIZE(params->reserved); ++i) 85 params->reserved[i] = 0; 86 return 0; 87 } 88 89 /* The state must not straddle a page, since pages can be zeroed at any time. */ 90 if (unlikely(((unsigned long)opaque_state & ~PAGE_MASK) + sizeof(*state) > PAGE_SIZE)) 91 return -EFAULT; 92 93 /* Handle unexpected flags by falling back to the kernel. */ 94 if (unlikely(flags & ~(GRND_NONBLOCK | GRND_RANDOM | GRND_INSECURE))) 95 goto fallback_syscall; 96 97 /* If the caller passes the wrong size, which might happen due to CRIU, fallback. */ 98 if (unlikely(opaque_len != sizeof(*state))) 99 goto fallback_syscall; 100 101 /* 102 * If the kernel's RNG is not yet ready, then it's not possible to provide random bytes from 103 * userspace, because A) the various @flags require this to block, or not, depending on 104 * various factors unavailable to userspace, and B) the kernel's behavior before the RNG is 105 * ready is to reseed from the entropy pool at every invocation. 106 */ 107 if (unlikely(!READ_ONCE(rng_info->is_ready))) 108 goto fallback_syscall; 109 110 /* 111 * This condition is checked after @rng_info->is_ready, because before the kernel's RNG is 112 * initialized, the @flags parameter may require this to block or return an error, even when 113 * len is zero. 114 */ 115 if (unlikely(!len)) 116 return 0; 117 118 /* 119 * @state->in_use is basic reentrancy protection against this running in a signal handler 120 * with the same @opaque_state, but obviously not atomic wrt multiple CPUs or more than one 121 * level of reentrancy. If a signal interrupts this after reading @state->in_use, but before 122 * writing @state->in_use, there is still no race, because the signal handler will run to 123 * its completion before returning execution. 124 */ 125 in_use = READ_ONCE(state->in_use); 126 if (unlikely(in_use)) 127 /* The syscall simply fills the buffer and does not touch @state, so fallback. */ 128 goto fallback_syscall; 129 WRITE_ONCE(state->in_use, true); 130 131 retry_generation: 132 /* 133 * @rng_info->generation must always be read here, as it serializes @state->key with the 134 * kernel's RNG reseeding schedule. 135 */ 136 current_generation = READ_ONCE(rng_info->generation); 137 138 /* 139 * If @state->generation doesn't match the kernel RNG's generation, then it means the 140 * kernel's RNG has reseeded, and so @state->key is reseeded as well. 141 */ 142 if (unlikely(state->generation != current_generation)) { 143 /* 144 * Write the generation before filling the key, in case of fork. If there is a fork 145 * just after this line, the parent and child will get different random bytes from 146 * the syscall, which is good. However, were this line to occur after the getrandom 147 * syscall, then both child and parent could have the same bytes and the same 148 * generation counter, so the fork would not be detected. Therefore, write 149 * @state->generation before the call to the getrandom syscall. 150 */ 151 WRITE_ONCE(state->generation, current_generation); 152 153 /* 154 * Prevent the syscall from being reordered wrt current_generation. Pairs with the 155 * smp_store_release(&_vdso_rng_data.generation) in random.c. 156 */ 157 smp_rmb(); 158 159 /* Reseed @state->key using fresh bytes from the kernel. */ 160 if (getrandom_syscall(state->key, sizeof(state->key), 0) != sizeof(state->key)) { 161 /* 162 * If the syscall failed to refresh the key, then @state->key is now 163 * invalid, so invalidate the generation so that it is not used again, and 164 * fallback to using the syscall entirely. 165 */ 166 WRITE_ONCE(state->generation, 0); 167 168 /* 169 * Set @state->in_use to false only after the last write to @state in the 170 * line above. 171 */ 172 WRITE_ONCE(state->in_use, false); 173 174 goto fallback_syscall; 175 } 176 177 /* 178 * Set @state->pos to beyond the end of the batch, so that the batch is refilled 179 * using the new key. 180 */ 181 state->pos = sizeof(state->batch); 182 } 183 184 /* Set len to the total amount of bytes that this function is allowed to read, ret. */ 185 len = ret; 186 more_batch: 187 /* 188 * First use bytes out of @state->batch, which may have been filled by the last call to this 189 * function. 190 */ 191 batch_len = min_t(size_t, sizeof(state->batch) - state->pos, len); 192 if (batch_len) { 193 /* Zeroing at the same time as memcpying helps preserve forward secrecy. */ 194 memcpy_and_zero_src(buffer, state->batch + state->pos, batch_len); 195 state->pos += batch_len; 196 buffer += batch_len; 197 len -= batch_len; 198 } 199 200 if (!len) { 201 /* Prevent the loop from being reordered wrt ->generation. */ 202 barrier(); 203 204 /* 205 * Since @rng_info->generation will never be 0, re-read @state->generation, rather 206 * than using the local current_generation variable, to learn whether a fork 207 * occurred or if @state was zeroed due to memory pressure. Primarily, though, this 208 * indicates whether the kernel's RNG has reseeded, in which case generate a new key 209 * and start over. 210 */ 211 if (unlikely(READ_ONCE(state->generation) != READ_ONCE(rng_info->generation))) { 212 /* 213 * Prevent this from looping forever in case of low memory or racing with a 214 * user force-reseeding the kernel's RNG using the ioctl. 215 */ 216 if (have_retried) { 217 WRITE_ONCE(state->in_use, false); 218 goto fallback_syscall; 219 } 220 221 have_retried = true; 222 buffer = orig_buffer; 223 goto retry_generation; 224 } 225 226 /* 227 * Set @state->in_use to false only when there will be no more reads or writes of 228 * @state. 229 */ 230 WRITE_ONCE(state->in_use, false); 231 return ret; 232 } 233 234 /* Generate blocks of RNG output directly into @buffer while there's enough room left. */ 235 nblocks = len / CHACHA_BLOCK_SIZE; 236 if (nblocks) { 237 __arch_chacha20_blocks_nostack(buffer, state->key, counter, nblocks); 238 buffer += nblocks * CHACHA_BLOCK_SIZE; 239 len -= nblocks * CHACHA_BLOCK_SIZE; 240 } 241 242 BUILD_BUG_ON(sizeof(state->batch_key) % CHACHA_BLOCK_SIZE != 0); 243 244 /* Refill the batch and overwrite the key, in order to preserve forward secrecy. */ 245 __arch_chacha20_blocks_nostack(state->batch_key, state->key, counter, 246 sizeof(state->batch_key) / CHACHA_BLOCK_SIZE); 247 248 /* Since the batch was just refilled, set the position back to 0 to indicate a full batch. */ 249 state->pos = 0; 250 goto more_batch; 251 252 fallback_syscall: 253 return getrandom_syscall(orig_buffer, orig_len, flags); 254 } 255 256 static __always_inline ssize_t 257 __cvdso_getrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state, size_t opaque_len) 258 { 259 return __cvdso_getrandom_data(__arch_get_vdso_rng_data(), buffer, len, flags, opaque_state, opaque_len); 260 } 261